Emacs is an advanced and extensible editor. An Emacs major mode
customizes Emacs for editing particular types of text documents. PSGML
is a major mode for SGML (a markup language) documents. PSGML
provides several functionalities for editing SGML documents:
indentation according to element nesting depth and identification of
structural errors (but it is not a validating SGML parser); menus and
commands for inserting tags with only the contextually valid tags;
attribute values can be edited in a separate window with information
about types and defaults; structure based editing includes movement
and killing; and also several commands for folding editing.